What happens if the landing gear on your flight breaks down? A pilot explains
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Last week, a Boeing 737 landing at Isfahan-Shahid Beheshti Airport (IFN) in Iran suffered the collapse of its left main landing gear shortly after touchdown. The aircraft skidded to a halt further down the runway and all passengers and crew were safely evacuated.
The failure of such a major part of the aircraft like this is rare and, as shown by the incident in Iran –even if it does happen – the outcome tends to be more damage to the aircraft than those inside it.
That said, like with all emergency situations that pilots could face, we practice these scenarios regularly in the flight simulator to ensure that we’re prepared should such a situation ever arise.
How the landing gear works
When the Wright brothers first took to the skies in 1903, their Wright Flyer didn’t even have landing gear – just a skid on which to touch down after their flight was launched from a track. As aircraft design progressed, it was agreed that adding wheels to the aircraft was of great benefit for a number of reasons, some of them immediately obvious.
Firstly, landing on skids is invariably bad for the aircraft structure. The high level of friction between the skids and the ground can cause the aircraft to come to an abrupt halt, causing damage to the airframe. By adding wheels, it enables aircraft to slow down much more slowly, allowing for the aircraft to be used again without the need for major rebuilding work.
The addition of wheels also meant that planes could be moved easily from one place to another, for example from the workshop to the area where it would take off. Once at this spot, it could then accelerate across the ground, removing the need for a long track to launch from.
The first aircraft with retractable gear
According to the Smithsonian Air and Space Museum, the first aircraft to have retractable landing gear was the Triad in 1911. This was an amphibious aircraft that could lift its wheels slightly to allow the aircraft to land on the water on its fitted skids. However, it wasn’t until the 1930s when real progress was made in retractable gear technology.
As aircraft design improved at a rapid rate, particularly when it came to aerodynamics, the large wheels hanging below the aircraft inflight became a hindrance. The benefits of the new streamlined body shapes were being held back by the landing gear sticking out into the airflow. This increased drag, reducing the top speed of the aircraft and increasing fuel usage.
What was needed was a way in which the landing gear could be used for take-off, but then once in the air taken out of the airflow. Then, when ready for landing, they could be put back into the airflow so the pilots could touch down safely. The answer to this problem was retractable landing gear which is still in use on aircraft today.
The Douglas SBD-6 Dauntless is credited to be the first high-performance aircraft with retractable landing gear. This enabled it to operate off aircraft carriers in WW2, making it a key asset in the Battle of Midway. Its design had the wheels folding inwards and being stowed in compartments in the belly of the aircraft. This gave the exterior of the aircraft a much improved aerodynamic shape, enabling the Dauntless to fly faster and for longer than any other comparable aircraft.
Modern Landing Gear
The landing gear on modern aircraft has come a long way since the days of WW2 and the Dauntless. Aircraft like the Boeing 787 Dreamliner have sophisticated landing gear that can take the impact of a landing with an aircraft weighing well over 250tons.
The landing gear system on the 787 Dreamliner consists of two main landing gear assemblies and one nose gear assembly. Each main gear set up has four wheels, each of which has an electronic brake. The nose gear has two wheels, neither of which have a brake.
To absorb the shock of the landing impact, the landing gear has an oleo strut, which acts as a type of suspension. This uses a mix of compressed air and hydraulic fluid to dissipate the forces experienced on landing across the airframe and also to dampen any recoil to reduce the chances of the aircraft bouncing back up into the air.
The landing gear lever in the flight deck is situated on the centre panel within easy reach of both pilots. To raise the gear, we simply move the gear lever to the up position and this starts the gear retraction sequence. Firstly, the gear bay doors start to open, creating space for the wheels to be retracted into the belly and nose of the aircraft. However, before this occurs, one other important thing must happen first.
On liftoff, the tyres will be spinning at around 180 mph. Bringing them up inside the aircraft at this speed could cause some serious vibration to be felt in the passenger cabin. As a result, before the wheels are folded away, the brakes on the main wheels are applied to stop them from spinning.
To lower the gear, pilots simply do the opposite. When the gear lever is moved to the down position, the gear bay doors open and the wheels free-fall out of their stowage without the use of the hydraulics system. When in the down position, it locks into position to stop them from folding on touchdown.
When all the gear is down and locked, the gear position indicator in the flight deck shows “down” in green.
What if the system fails?
In over 15 years of flying, I have never had a problem with the landing gear. That said, I have practised multiple times in the simulator what to do should this unlikely event occur.
The most important factor in this scenario is time. How much time do we have to deal with the situation this is almost always determined by the amount of fuel onboard.
In June last year, a 787 got airborne from Sydney, Australia bound for Perth. Shortly after the crew moved the landing gear lever to the UP position, they received a message from the aircraft’s centralised warning system that there was a problem.
“GEAR DISAGREE” is displayed on our screens when the actual position of the gear is different to the position of the landing gear lever. However, as we are unable to physically see what is going on, we have to rely on the indications in the flight deck.
What this crew would have seen was that the nose gear had retracted as normal, but the two main landing gear were still in the down position. What the aircraft doesn’t tell us is why this has happened. The hydraulic actuators could have failed, there may be something obstructing the gear from moving or it could be something else.
What we have to do is complete the GEAR DISAGREE checklist and then come up with a plan of when and how to land the aircraft.
The good thing about this happening just after take-off is that we will have plenty of fuel which in turn means plenty of time. The downside is that with the gear sticking out into the airflow is the extra drag. Not only does this double our fuel usage, it will also affect our rate of climb — something which we must let ATC know as soon as is practicable.
The increased drag from the wheels is also quite noisy. The faster we go, the noisier it gets. Not only is this uncomfortable for the passengers, but it can also be quite a distraction in the flight deck. As a result, we need to decide the best course of action for the situation.
If we accelerate and retract the flaps, there will be less drag on the aircraft reducing the fuel burn but the downside is that it will be quite noisy, particularly if the aircraft is heavy as we will need to be flying faster.
The other option is to keep the flaps out so that we can keep the speed slower, but this of course increases the fuel usage. What a crew decides to do on the way would be up to them and what they deem best at that moment.
With the checklist complete, we would then need to decide what we’re going to do.
If two gear are stuck down and one has retracted fine, the chances are that the retracted gear will come back down as expected. As a result, we can expect a normal landing.
The most likely cause for this, like in the Sydney event, is the gear pins being left in by the ground crew.
When towing an aircraft to or from the gate, the ground crew insert locking pins into the landing gear to stop them from inadvertently retracting. Once the aircraft is parked up, the pins should then be removed.
If they are not, when the pilots go to retract the gear, the pins do exactly the job they are meant to do and stop the gear from retracting.
If the GEAR DISAGREE message appears on landing, things are a little more serious. Not because it’s any more of an issue, but because the time available is much less.
When determining the fuel needed before departure, we must have enough so that we arrive with fuel to divert to another airfield and also have our ‘final reserve’ fuel intact — enough for 30 minutes flying time. This is to cover any last-minute unforeseen circumstances that may arise.
When we put the gear down, just a few minutes from landing, the chances are that we will have a little more than the diversion and reserve fuel left. Ideally, we don’t want to cut into our final reserve fuel so, if the weather at the destination is good enough, we can use the diversion fuel to sort the issue with the gear.
The 2nd part of the GEAR DISAGREE checklist is designed to lower the gear with the alternate system. This manually releases the locks keeping the landing gear and doors in place, allowing the wheels to free fall down into position.
If this doesn’t work, we are in a more serious situation.
Landing with partial gear
If by this point one of the gear still won’t come down, we need to prepare to land with whatever gear we do have in the correct position.
As mentioned before, time will be tight with the amount of fuel remaining and we may end up using our final reserve fuel — exactly the kind of scenario it is designed for.
We will inform the cabin crew who will prepare the cabin for an emergency landing. They will remind the passengers of the brace position and how to evacuate the aircraft if necessary. We will also inform ATC who will, in turn, notify the airport fire services who will take up tactical positions along the runway, ready to provide assistance when we land.
With the cabin and airport services prepared, it is time for us to make the landing.
The aim is to keep the aircraft up on whatever wheels are available for as long as possible. For example, if the nose wheel is stuck up, we will hold the nose off the ground for as long s possible. If the left main gear is stuck up, we will hold the left wing off the ground as long as we can.
Eventually, one part of the aircraft will hit the runway, be it the nose or an engine. Once we have come to a stop, we then have a decision to make. Is it safer to keep everyone on board the aircraft or is it safer to initiate an evacuation?
To evaluate the situation, we will use the radio to speak to the fire services who will be outside the aircraft and in the best position to assess the damage. We will also speak to the cabin crew to determine the situation in the cabin.
If there are no signs of fire or smoke, most of the time it is safer to keep everyone on board until they can exit the aircraft safely via steps or some other means.
The chances of the landing gear getting stuck in the wrong position are incredibly rare on a modern aircraft. The most likely cause is that the gear pins were left in after the aircraft was towed to the gate which then prevent the gear from retracting.
However, if the gear does fail to deploy, pilots are well practised in how to handle the situation. By going through in-depth checklists and procedures, we can set the aircraft up for a non-normal landing which may not even require an emergency evacuation.
That said, knowing what to do in the event of an unplanned emergency is key and it is why I always urge people to watch the safety demonstration on every flight. Even if you may have seen it before, as we do in the flight deck, refreshing that important information before each flight could make all the difference.
Featured Image – Getty Images
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